Core-Shell Dendrimers

By covalent linkage of different types of molecules it is possible to obtain materials with novel properties that are different from those of the parent compounds. Examples of such materials are block-copolymers, soaps, or lipids which can self-assemble into periodic geometries with long-range order. Due to their amphiphilic character, these molecules tend to micellize and to phase-separate on the nanometer scale. By this self-assembly process the fabrication of new na-noscopic devices is possible, such as the micellization of diblock-co-polymers for the organization of nanometer-sized particles of metals or semiconductors [72 - 74]. The micelle formation is a dynamic process, which depends on a number of factors like solvent, temperature, and concentration. Synthesis of micelles which are independent of all of these factors via appropriately functionalized dendrimers which form unimolecular micelles is a straightforward strategy. In 

Up to now only amphiphilic dendrimers with soft cores and soft shells were used. Unfortunately the shape and the size of this type of dendrimer also show a dependence on the factors mentioned above [75-77]. By using dendrimers with a hard inner core it would be possible to study the self-organization of amphiphilic dendrimers as related to shape and size of the core. 

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Fig.8 Stoichiometric assembly of an octa-cationic core-shell dendrimer and eight anionic Pd(II) guest molecules...

Fig. 1 Dendritic core-shell star polymers, (a) Dendritic core-shell star polymers of different architectures with varying densities of the polycationic polymer chains grafted from the dendrimer core as well as varying numbers of positive charges along the individual polymer chains, (b) Cellular uptake of selected macromolecules by ECV-304. The bars represent relative fluorescence units (RFU) measmed in individual cells. Data represent mean values ( SEM after an incubation periods of 6 and 24 h. (c) Cell uptake of polycationic core-shell dendrimers into ECV-304 cells after 15 min. ECV-304 were stained using a green fluorescence cell tracker, whereas the core-shell macromolecules are shown with a red color originating from the PDI core...

Figure 4 Core-shell dendrimer architecture with mathematics defining number of surface-groups (Z), number of branch cells (BC), theoretical molecular weights (MW), dimensions (A) as a function of generation (G), branch-cell multiplicity (A/ ), core multiplicity (Afc), and core molecular weight (Me), (a) alkylation chemistry, (b) amidation chemistry.

Catino EV, Crooks RM (2011) Characterization of Pt Cu core shell dendrimer-encapsulated nanopeuticles synthesized by Cu underpotential deposition. Langmuir 27 4227... 

Another system based on electrostatic interactions has been reported by van Koten and coworkers [36]. A core-shell dendrimer with a cationic tetra[bis(benzyIammonium)aryl]silane core has been used as a template for the... 

Synthetic pathways have been deliberately aimed at producing megamers. For example, poly(amido amine) dendrimers of different generations have been combined to give well-defined core-shell megamers in which a central, large core dendrimer is surrounded by a well-defined number of smaller dendrimers. 

Similarly, Pd, Ag, and Pd-Ag nanoclusters on alumina have been prepared by the polyol method [230]. Dend-rimer encapsulated metal nanoclusters can be obtained by the thermal degradation of the organic dendrimers [368]. If salts of different metals are reduced one after the other in the presence of a support, core-shell type metallic particles are produced. In this case the presence of the support is vital for the success of the preparation. For example, the stepwise reduction of Cu and Pt salts in the presence of a conductive carbon support (Vulcan XC 72) generates copper nanoparticles (6-8 nm) that are coated with smaller particles of Pt (1-2 nm). This system has been found to be a powerful electrocatalyst which exhibits improved CO tolerance combined with high electrocatalytic efficiency. For details see Section 3.7 [53,369]. 

Each step in dendrimer synthesis occurs independent of the other steps therefore, a dendrimer can take on the characteristics defined by the chemical properties of the monomers used to construct it. Dendrimers thus can have almost limitless properties depending on the methods and materials used for their synthesis. Characteristics can include hydrophilic or hydrophobic regions, the presence of functional groups or reactive groups, metal chelating properties, core/shell dissimilarity, electrical conductivity, hemispherical divergence, biospecific affinity, photoactivity, or the dendrimers can be selectively cleavable at particular points within their structure. 

The chemical composition of particles can be just as varied as their shape. Commercial particles can consist of polymers or copolymers, inorganic constructs, metals and semiconductors, superparamagnetic composites, biodegradable constructs, and synthetic dendrimers and dendrons. Often, both the composition of a particle and its shape govern its suitability for a particular purpose. For instance, composite particles containing superparamagnetic iron oxide typically are used for small-scale affinity separations, especially for cell separations followed by flow cytometry analysis or fluorescence-activated cell sorting (FACS). Core-shell semiconductor particles, by... 

In contrast to traditional polymers, dendrimers are unique core-shell structures possessing three basic architectural components namely, (I) a core, (II) an interior of shells (,generation) consisting of repetitive branch cell units and (III) terminal functional groups (i.e. the outer shell or periphery) as illustrated in Figures 1.13 and 1.14. 

Figure 1.14 Three-dimensional projection of dendrimer core-shell architecture for G = 4.5 poly(amidomine) (PAMAM) dendrimer with principal architectural components (I) core, (II) interior and (III) surface...

More recently, mathematically defined, structure controlled, covalent megamers have been reported. They are a major subclass of megamers also referred to as core-shell tecto dendrimers) [126-128], Synthetic methodologies to these new architectures have been reported to produce precise megameric structures that adhere to mathematically defined bonding rules [91, 129], It appears that structure controlled complexity beyond dendrimers is now possible. The demonstrated structure control within the dendrimer modules, and now the ability to mathematically predict and synthesize precise assemblies of these modules, provide a broad concept for the systematic construction of nanostructures with dimensions that could span the entire nanoscale region (Figure 1.24). 

Figure 6.7 Illustration of a core-shell tecto(dendrimer) partial shell and saturated shell models.

Table 10.1 Core-shell tecto(dendrimer) molecular weight analysis results obtained from MALDI-MS, AFM and PAGE.

Figure 12.20 Schematic representation of a core-shell tecto-(dendrimer) molecule in solution...

ATOMIC FORCE MICROSCOPY FOR CHARACTERIZATION Table 12.2 The molecular size of the core-shell tecto-dendrimers... 

Core-shell tecto-dendrimers Theoretical calculations AFM measurement ... 

Tomalia et al. reported that the surface amines of PAM AM dendrimer can successfully react with methyl esters of other PAMAM dendrimers to afford core-shell tecto-(dendrimer) molecules [65]. Furthermore, they also reported the synthesis of rod-shaped cylindrical dendronized polymers from poly(ethyleneimine) cores without any crosslinking, albeit with the use of excess reagents [58]. These reports lead us to propose a new approach toward hybridized dendrimers and polymers (path C, Figure 15.3). As shown in Scheme 12,... 

Figure 16.11 The pH-dependent conformational behaviour of polypropylene imine) dendrimers. At low pH (left) the occurrence of a soft-core, dense-shell dendrimer, whereas at high pH (right) severe back-folding occurs leading to a dense-core structure [57]...

Laboratory Synthesis and Characterization of Megamers Core-Shell Tecto(dendrimers)... 

Figure 27.1 Hierarchy of empirical construction components (A) monomers, (B) branch cells, (C) dendrons and (D) dendrimers leading to (E) core-shell tecto(dendrimers)...

Laboratory procedures are presented for two divergent approaches to covalent structure controlled dendrimer clusters or more specifically - core-shell tecto(dendrimers). The first method, namely (1) the self assembly/covalent bond formation method produces structure controlled saturated shell products (see Scheme 1). The second route, referred to as (2) direct covalent bond formation method , yields partial filled shell structures, as illustrated in Scheme 2. In each case, relatively monodispersed products are obtained. The first method yields precise shell saturated structures [31, 32] whereas the second method gives semi-controlled partially shell filled products [30, 33],... 

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